2122
Ind. E n g . C h e m . R e s . 1987, 26, 2122-2129
Synthesis of Alcohols from Carbon Oxides and Hydrogen. 4. Lumped Kinetics for the Higher Alcohol Synthesis over a Zn-Cr-K Oxide Catalyst E n r i c o Tronconi,* Natale Ferlazzo, Pi0 F o r z a t t i , and I t a l o P a s q u o n Dipartimento di Chimica Industriale ed Ingegneria Chimica "G. N a t t a " del Politecnico, I-20133 Milano, I t a l y
The kinetics of the synthesis of methanol and higher alcohols from syngas over a K-promoted Zn-Cr oxide catalyst have been investigated, exploring the effects of space velocity and reactor feed composition. On the basis of a simplified reaction pattern involving lumping of alcohols and hydrocarbons heavier than C1, a n isothermal kinetic model has been developed which adequately describes the experimental data. T h e model predicts t h e apparent inhibiting effect of COz when added to the feed mixture and explains the observed occurrence of a maximum in the yield of higher alcohols when varying the Hz/CO feed ratio. Some related process implications are also discussed. In the last few years, the Chemical and Petroleum Industry has shown growing interest in the use of higher aliphatic alcohols as components for alcohol-gasoline motor fuel blends. When compared to pure methanol, alcohol mixtures exhibit better volatility, solubility, and water tolerance characteristics. Besides, the increasing restrictions on the permissible levels of tetraethyllead in gasoline make higher alcohols very attractive, in perspective, as gasoline improvers, due to their property of being effective octane boosters. These considerations have initiated the reinvestigation of the already well-known direct synthesis of alcohols from carbon monoxide and hydrogen (Frohlich and Lewis, 1928; Frohlich and Cryder, 1930; Graves, 1931; Morgan et al., 1932; Anderson et al., 1952; Runge and Zepf, 1954; Natta et al., 1957). Research activities in this field are presently under way both in the academic world and by several industrial companies. The industrial interest is readily appreciated on inspecting the patent literature issued since the end of the last decade. Among others, Snamprogetti has developed the production of alcohol mixtures from syngas up to an industrial scale (Paggini and Sanfilippo, 1986). IFP has disclosed a process to produce primary straight-chain alcohols over Cu-Co-based catalysts promoted by alkali metals (Courty et al., 1984; Ohno et al., 1986). DOW Chemical and Union Carbide have independently applied for patents concerning processes based on modified molybdenum sulfide catalysts and in 1985 have joined forces to commercialize their findings (Quarderer, 1986). LURGI has recently presented a new technology for mixed alcohols production using copper-based catalysts (Supp, 1986). Research projects on the higher alcohol synthesis appear concerned primarily with the characterization and the development of active catalysts for this reaction, with the identification of its chemical mechanism, and are often aimed at rationalizing the product distribution in the alcohol mixture (Pillai et al., 1981; Courty et al., 1982; Hofstadt et al., 1982; Greene, 1982; Smith and Anderson, 1983; Vedage et al., 1983; Smith and Anderson, 1984; Mazanek, 1986). To our knowledge, however, no systematic study of the overall kinetics has been reported so far, though a number of observations about the effects of some operating parameters (e.g., feed composition, reaction temperature, space velocity) can be found in the existing literature. With respect to this point, the pioneering work by Frohlich and co-workers ought to be mentioned
* To whom correspondence should be addressed.
(Frohlich and Cryder, 1930): on outlining a possible reaction scheme involving chiefly the successive condensation of lower alcohols, these authors first demonstrated the relevance of the water gas shift reaction to the overall synthesis process. Catalysts utilized in tho coproduction of methanol and higher alcohols from CO-.Hz mixtures generally fall into one of the three classes listed below. (a) The first class is high-temperature methanol-synthesis catalysts (Zn-Cr oxides) modified with alkali promoters: these catalytic systems are typically operated around 400 "C and up to 400 atm and yield preferably branched alcohols, among which isobutyl alcohol is the main component. (b) The second class is alkali-promoted low-temperature Cu-based methanol catalysts operating at about 300 " C and pressures of 100 atm or less. Such systems exhibit a similar product distribution but are possibly affected by sintering of copper. (c) The third class is modified Fischer-Tropsch catalysts, whose hydrogenating action has been moderated by addition of metal sulfides, borates, and phosphates and/or by addition of alkali promoters (Taylor, 1934). Typical products of these catalytic systems include primary linear alcohols following a Schultz-Flory molecular weight distribution. Yet, the parallel formation of hydrocarbons can hardly be reduced to acceptable levels. Formation of higher alcohols has been observed also on platinum-group metals (Klier, 1980; Pasquon, 1984), but the productivity is very low in this case. Thus, at the present state of technology, the catalysts belonging to class a appear as those with the most consolidated background. As a further step in a research activity concerning the alcohol synthesis (Forzatti et al., 1984; Villa et al., 1985, 1986), in this paper we present the results of a systematic isothermal kinetic study on the combined synthesis of methanol and higher alcohols over a Zn-Cr oxide commercial catalyst promoted with KzO. This investigation was designed to gain insight into the complexities of the reaction scheme; its scope is the development of a simplified kinetic model, which is, however, suitable for rationalizing the observed effects of the process variables under study.
Experimental Section Apparatus. The kinetic study was performed by using a tubular fixed-bed integral reactor (10-mm inner diameter) made of INCONEL 600, a Ni-Cr-Fe alloy. The inner
0888-5885f 87 f 2626-2122$01.50f 0 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2123 wall of the reactor was lined with copper in order to prevent Fe from catalyzing the formation of hydrocarbons. The reactor was immersed in a thermostatic bath of sand fluidized by compressed air controlled by an electronic temperature regulator and controller. The temperature a t different axial positions along the catalyst bed was measured by means of a thermocouple which could slide inside a capillary tube immersed in the catalyst bed. The catalyst was placed on a copper porous plate between two layers of carborundum, and the gas flow was fed from the top. The gaseous feed composed of calibrated mixtures of CO, COz, H2, and He was supplied from cylinders to a compressor loop including two equal tanks and a diaphragm compressor and then to the reactor. Helium was added to the reagents as an internal gas chromatographic standard. Two emergency relief valves, with pressure set a t 110 atm, were placed in the compressor loop and upstream of the reactor inlet to prevent uncontrolled pressure rise in the system. The effluent from the reactor flowed in traced lines to a condenser (T= -50 "C), where the condensable reaction products were separated from the gaseous products and the unreacted reagents and collected in a small stainless steel cylinder. The duration of a standard run was typically between 4 and 8 h, depending on the feed rate, so that a few grams of liquid products could be collected. The samples of the effluent gas stream were periodically analyzed on line by a 5750 G Hewlett-Packard gas chromatograph, using both TC and FI detectors. A Porapak QS (100-120-mesh) column (1/8-in. diameter, 4.8-m long) was used and operated isothermally at 75 "C. This allowed separation of He, CO, CH,, C02, ethane, ethylene, and HzO. The outlet concentrations of the gaseous products were obtained by averaging the gas-chromatographic readings over the whole run. Higher hydrocarbons could not be analyzed on line. During some of the runs, however, samples of the effluent gases and of the vapors of the liquid products were collected and later analyzed specifically for identification of the hydrocarbons produced in the synthesis reaction. These were detected in amounts that were not negligible. Samples of the liquid products were injected for analysis in a Varian 3400 gas chromatograph equipped with a TCD. Separation was effected by a Porapak QS (100-120-mesh) column (1/8-in. diameter, 3.30-m long) using a proper temperature program which allowed identification of up to c8 products. Catalyst. The catalyst utilized in the investigation was prepared from a commercial Zn-Cr oxide. The catalyst was ground to a size of 14-20 mesh; then it was prereduced in a flowing N2-H2 stream according to the following schedule: heating to 300 OC in 5% H2 (heating rate 100 OC/h); heating to 400 "C, with H2 concentration progressively increased from 5% to 100% (heating rate 40 OC/h, hold 30 min); cooling down to room temperature. The catalyst was then impregnated with an amount of CH,COOK corresponding to 3% K 2 0 by weight, left in air for 30 min a t room temperature, and finally exiccated in oven a t 110 "C for 1day. The final catalyst activation was performed in the reactor by slowly raising the temperature up to the catalyst working temperature (400 "C) in a flowing H2-N2 stream, the H2 content being progressively increased from 2% up to 100%. The procedure lasted 6 h. After activation, the catalyst was always kept under reaction mixture or inert atmosphere until the kinetic study was completed. The main catalyst characteristics are listed in Table I.
Table I. Catalyst Properties fresh catalyst composition apparent density, g/cm3 BET surface area, m2/g pore vol, cm3/g K 2 0 content after reaction: BET surface area, m2/g phases identified catalyst load, g
Zn3Cr, 1.46 130 0.2 3.0% by w t 41
ZnO, ZnCr20, 11
Table 11. Composition of the Condensed Products Obtained at Standard Reaction Conditionsa % by wt % by wt identified of total of total sDecies lia identified sDecies lia H2O CHBOH CH3CHO CZHbOH C2H5CHO i-CSH70H wC~H~OH i-CSH7CHO
2' = 400 "C, = 1.1.
7.84 41.41 0.47 1.40 0.62 0.27 3.76 2.80
i-CdHgOH n-C,HgOH CC5-OH ZCe-OH CC7-OH ECB-OH
other oxygenated compds
P = 87 atm,
-2582 0.26 4.60 2.98 4.39 3.00 0.38
= 2.7 mol/h, CO? = 0%, (H2/CO)0
Experimental Program. All the kinetic runs were performed a t T = 400 OC and P = 87 atm. The effects associated with the following operating variables were explored: feed flow rate (range: 1.35-4.94 mol/h); C 0 2 % volume in the feed (range: 0-5.93%); H2/C0 feed molar ratio (range: 0.55-11.9). In preliminary tests, the catalytic activity appeared to be slightly improving, but later on it remained substantially stable during the bulk of experiments, as demonstrated by replicated runs a t standard conditions. The contribution of the empty reactor to hydrocarbon formation was found to be negligible. Results Reaction Scheme. A broad spectrum of products was always observed during the reaction of carbon monoxide and hydrogen. Methanol, water, carbon dioxide, hydrocarbons, and CZ-cS linear and branched alcohols were recognized as the main reaction products. Among the higher alcohols, isobutyl alcohol was present in the greatest amounts. Aldehydes and traces of ketones and esters were also detected. A typical distribution of the condensed products is given in Table 11. A reaction network taking all of these products into account seems unpractical, as it would require far too many kinetic parameters even if the simplest rate equations for the individual reaction steps were assumed. Also, computational difficulties are likely to arise when estimating the parameters in this case. Moreover, this kind of kinetics would be of doubtful use for process design, since the main effects associated with the operating variables could hardly be isolated. On the other hand, for application purposes the reaction products need to be characterized in terms of the overall and relative content of methanol and higher alcohols, which primarily determines the properties and the value of the end product, of water, which affects the separation costs of the reactor effluents, and of hydrocarbons, which are responsible for losses of carbon to undesired products and for problems associated with their buildup in the synthesis loop. Accordingly, in this work we adopt the following lumped simplified reaction scheme:
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Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
GO CHSOH
+ 2H2
-
CH,OH
+ ( N , - 1)CO + 2(Nc - 1)H, CO
+ H,O
GO
+ 3Hz HA
+
*
HA
OL
-+
+ ( N , - 1)HzO (2)
C 0 2 + H,
CHI
+ HzO
+ H,O
(1)
(3) (4) (5)
Reactions 1 and 3 represent the methanol synthesis and the water gas shift reaction, respectively, and describe the observed formation of methanol and carbon dioxide. Thermodynamic analysis showed that both reactions approached chemical equilibrium in our experimental conditions. The chain growth from methanol to higher alcohols is schematized by reaction 2. Here the higher oxygenated products have all been lumped into HA, a pseudocomponent representing an aliphatic alcohol with N , carbon atoms. N , is defined as the carbon number averaged over all the higher oxygenated reaction products. Although the outlet concentration of higher alcohols varied by almost an order of magnitude during the runs, the values of N , calculated from the reaction products ranged only between 4.0 and 4.5, with an average value of about 4.2 that was assumed in the calculations. Noticeably, reaction 2 does not necessarily imply that methanol itself is the true intermediate in the higher alcohol synthesis. Such a role, however, is likely played by a methanol-related C1 adsorbed species. Reaction 4 describes the observed formation of methane. Reaction 5 has been introduced to account for the formation of Cs+ hydrocarbons. As discussed in the Experimental Section, hydrocarbons higher than ethylene and ethane could not be observed quantitatively during the kinetic runs. Nevertheless, such products were monitored by watching the “excess water” DH20, i.e., the water formed in addition to that expected according to the stoichiometry of reactions 1-4 alone. DHzO is calculated from the H20 balance. DH20 = H20
+ ( ( 2 0 2 .- COZ’)
- ( N , -. 1)HA - CHI
Reaction 5 assumes that the formation of higher hydrocarbons occurs by dehydration of the pseudoalcohol HA to the corresponding pseudoolefin OL having the same carbon number as HA. The average carbon number of the higher hydrocarbons estimated from analysis of the effluent gas samples collected during some of the runs was indeed found close to that of the higher alcohols. Calculations of chemical equilibria confirm that the formation of olefins via dehydration is a possible reaction in our conditions, and indeed this is a known secondary reaction of higher alcohols (Frohlich and Cryder, 1930; Natta et al., 1957). In partial support of this assumption, we remark that a straight line results from plotting DHzO against W(HA)l’z/P (see Figure l),which indicates that the rate of excess water formation exhibits the same square-root dependence on PHArecognized for the rate of alcohol dehydration over oxide catalysts (Figueras et al., 1971). Nonetheless, on the basis of our data, one cannot rule out the possibility that higher hydrocarbons are formed by different mechanisms (e.g., including a Fischer-Tropsch synthesis), so that reaction 5 should be regarded a t this stage as an empirical description of the formation of C,, hydrocarbons which is consistent with thermodynamics and kinetics within a lumped global approach. The detailed study of the distribution of higher alcohols and higher hydrocarbons among the reaction products, and
i/. Figure 1. Characteristic kinetic plot for the rate of formation of DH20, the amount of water unexplained by reactions 1-4 alone.
I , , V
05-
,.
4-
2
4
6
b
10
W,/Fo, g h l m o l
Figure 2. Effect of contact time on the observed ethane/ethylene ratio.
Figure 3. Effect of H, feed concentration on the observed ethane/ethylene ratio.
the discussion of the underlying mechanisms, will be addressed in a future paper.
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2125 Figures 2 and 3 show that the ratio C2H6/C2H4grows with both the contact time and with the hydrogen partial pressure and suggest that, once they are formed, olefins can then undergo a consecutive hydrogenation. Such a picture is consistent with thermodynamics. This reaction, however, is responsible only for a minor Hz consumption and does not affect the primary reaction products. Thus, we will not consider the hydrogenation of olefins in the following treatment. Kinetic Model. Reactions 1-5 provide a lumped reaction scheme involving a total of six reaction products, namely, CH30H, C02,HzO, HA, CH,, and OL. Concerning the rate equations for the five individual reaction steps, the following expressions have been assumed: rl
rZ r3
= kl(PCOPH22 - PCH30H/K1)
(6)
=
(7)
k2kH30H/(1
-t KwPH20)
= k3(PCOPH20 - PCOgHl/K3) r4
7-5
=
(8)
(9)
k&H2
= MPH.J"~
(10)
Rate equations 6 and 8 account for the reversible nature of the methanol synthesis and of the water gas shift reaction. Equation 6 is a simplified form of a well-established kinetic expression (Natta et al., 1955), from which it was derived by dropping the Langmuir-type adsorption term appearing in the denominator. Equation 8 has been proposed by Moe (1962) to describe the rate of the shift reaction. Rate equation 7 relies on two assumptions: (a) the rate of formation of all the higher oxygenated products depends on the methanol partial pressure; (b) water vapor acts as a strong inhibitor. In their early work on the synthesis of higher alcohols over a Zn-Mn-Cr-K oxide catalyst, Frohlich and Cryder (1930) showed that methanol and ethanol behave as intermediate products, the overall kinetics being controlled by the initial conversion of methanol to ethyl alcohol. In fact, according to proposed chain growth schemes (Smith and Anderson, 1984; Sanfilippo, 1986), the C1 C2 step is controlled by a slow a-addition, whereas successive chain growth may occur mainly via much faster additions at the fl-carbon atom of the growing alcohol. Assumption a is in line with this picture. Indeed, assumption a is consistent with mechanisms whose rate-determining step involves the transformation of an intermediate adsorbed C1 species in equilibrium with gaseous methanol. Frohlich and Cryder reported also experimental evidence for the inhibiting effect of water. For example, some of their results show that the addition of water to a reacting mixture of methanol and hydrogen reduces by 90% the yield of higher alcohols as compared to that obtained from a mixture of methanol and H2 alone in the same conditions. A feed mixture of carbon dioxide and hydrogen reportedly yielded methanol but no higher alcohols because the water formed as a primary product in the reaction C 0 2 + 3H2 CH,OH + H 2 0 totally inhibits the formation of higher alcohols from methanol. The inhibition may result from the competitive adsorption of water and alcohols, particularly methanol, on the catalyst and is therefore expected to affect primarily a chain-growth mechanism controlled by the surface concentration of methanol or methanolrelated species, in agreement with assumption a. In rate equation 7, the inhibiting effect of water has been empirically introduced by means of a Langmuir-type adsorption term in the denominator of the right-hand side.
Table 111. numerical const used in the calculations N = 4.2 K1 = 1.57 X atm-2 K , = 14.8 parameter estimates k1 = 4.3 X mol g-' h-' atm-3 k z = 3.0 X mol g-' h-' atm-' k , = 1.3 X mol g-' h" atm-2 k4 = 1.0 X mol g-' h-' atm-' k6 = 9.7 X mol g-' h-' atm45 K , = 8.84 atm-' indexes for model adequacy MSE = 2.557 X response mean 70 error unexplained error variance linear correlation index
COZ 8.50
CHSOH HA 9.29 7.94
0.098 0.10 0.994 0.862
CH4 15.0
H20 16.0
0.023 0.036 0.051 0.978 0.965 0.915
Equation 9 assumes that the rate of methane formation depends linearly on the hydrogen partial pressure. This choice is in line with literature data showing that methanation depends primarily on the Hz concentration (Henrici-Oliv6 and Oliv6, 1984). Finally, eq 10 attributes to the alcohol dehydration reaction a typical kinetic expression according to literature results (Figueras et al., 1971), neglecting saturation effects. Reactor Model. Computations along the lines suggested by Mears (1971) confirmed that the heat and mass transport across the film was fast compared to the reaction kinetics. Likewise, limitations due to pore diffusion could be ruled out. A minor axial T gradient was detected (5-7 "C at the highest conversions), but the reactor has been regarded as isothermal anyway, in line with the degree of approximation involved in the whole analysis. Based on the above observations, the following monodimensional, pseudohomogeneous plug-flow reactor model has been assumed: ~ S C H , O H / ~ ( W ~=/ J71@- )r2 dtHA/d(Wc/J@) = r2 - r5 dSco2/d(Wc/J@;o) = r3
-+
-
dECH,/d(Wc/F) = r4 dEoL/d(Wc/J") =
rj
(11)
where Si = F i / F , and the rate expressions are given by eq 6-10. The following initial conditions apply: at W c / F = 0 , SCH~OH= SHA = ~ C H ,= ~ O = L 0 ; and Sco2= Fco21J@. Calculation of the composition at the reactor outlet then requires the numerical integration of the five ODES above, eq 11,coupled with atomic balances for C, H, 0, and He. A routine specially developed for "stiff" applications (Hindmarsh, 1980) has been employed for this purpose. The constants K1 and K3 appearing in eq 6 and 8 are related to the true reaction equilibrium constants Keq1and Keq3via the fugacity ratios KY1and Ky3,
K = Keq(T)/Ky(TP)
(12)
K 1 and K , at T = 400 "C and P = 87 atm have been computed from standard thermodynamic data and from expressions for K , reported by Klier et al. (1982). Their values are given in Table 111. Parameter Estimation. The six kinetic parameters (kl-k5, K,) appearing in eq 6-10 have been estimated by the least-squares method. The experimentally dependent variables were the measured outlet mole fractions of the five reaction products observed, CO,, CH,OH, HA, CH,,
2126 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
I
I
41
% co; 0 (HZ/CO)O=l 1 6 con CH30H v HA 0 CHI. H20
11-11
(HZ/CO)"= 1 1 Wc/Fo:41 g hlmol A ACOz CH30H v HA o CH4 H20
15
,
,
2
I
6
4
co;
W, / F', g h /mol
'1.
Figure 4. Kinetic runs: effect of W,/F'. Measured outlet concentrations of the observed reaction products and model fit.
Figure 5. Kinetic runs: effect of feed COz content. Measured outlet concentration of the observed reaction products and model fit. ACOz is the concentration of formed COz.
and H 2 0 (Yij),while the model responses were the mole fractions ofi the same species calculated from integration of eq 11 (Yij). Due to the nonquantitative analysis of higher hydrocarbons, carbon balance requirements could not serve as an approximate constraint. The parameter estimates have been obtained by minimizing the sum of squares of weighted residuals over all responses,
Use of eq 13 implies that the covariances between the responses within each experiment are negligibly small: this is consistent with the adopted analytical procedure and with the separation system exhibiting no overlapping gas-chromatographic peaks. Thirteen runs have been considered associated with the effects of feed flow rate, C02feed content, and H,/CO feed molar ratio. A specific, efficient computer program has been employed to perform the multiresponse nonlinear regression (Villa et al., 1985). The calculated estimates of the kinetic parameters are listed in Table 111along with the constants used in the kinetic analysis and some indexes of the agreement of the model with the experimental data. The standard deviations for the parameter estimates were typically large, out of two reasons: (a) the experimental program included only 13 runs with a limited representation of the variable space; (b) reactions (1)and (3) were always fast in approaching thermodynamic equilibrium, so that the corresponding rate constants were ill-determined. For atl responses, however, the mean percent error appears comparable to the experimental error evaluated from genuine replicated runs. Measured outlet percent mole fractions of the observed reaction products are compared with computed results in Figures 4, 5 , and 6. The curve fit, taken together with the analysis of residuals and the statistical indexes of Table 111, indicates that the kinetic model based on eq 6-10 provides a representation of the rate data which is adequate to the degree of approximation of the whole approach over the region investigated. It is worth noticing that the methods presented here are applied to the interpretation of data from an industrial-type laboratory and are aimed primarily at ra-
2.0- A
,
4
,
8
1
12
I
( HzlCO)"
Figure 6. Kinetic runs: effect of H2/C0 feed molar ratio. Measured outlet concentrations of the observed reaction products and model fit.
tionalizing the effects associated with the investigated process variables.
Discussion Effect of W,IFa. The data in Figure 4 demonstrate that methanol formation is fast compared to the synthesis of higher alcohols and is controlled by chemical equilibrium. The slight decline of the methanol yield corresponding to prolonged residence times results from the enhanced chain growth to higher products. Effect of C 0 2 Feed Content. At the conditions of Figure 5 , the addition of 5.93% C02 to the reaction feed mixture depresses the yield of higher alcohols by almost a factor of 3. Methanol formation is marginally affected, however, so that the relative content of HA with respect
Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987 2127
co;
= 0.
0
1
2
3
L
5
K W P YC0,’KJ
-
Figure 8. Calculated optimum H 2 / C 0 ratio for the rate of higher alcohol formation according to eq 7, as a function of H 2 0 inhibition.
1
2
3
4
5
6
7
8
N
”
1
6
4
1
co;
2
3
4
5.93 %
5
6
7
8
N
Figure 7. Schultz-Flory plots of the alcohol molecular weight distribution. WJEd = 4.1 mol/(g h); (H2/C0)0 = 1.1; CO; = 0% (a) and 5.93% (b).
to methanol is consequently reduced. These effects appear to be well explained by the inhibiting action of water assumed in formulating the rate equation for the higher alcohol synthesis, eq 7. The product distribution is also strongly influenced by the presence of carbon dioxide in the feed. Parts a and b of Figure 7 compare the observed molecular weight distributions of the alcohols obtained with no COz and with 5.93% COz added to the feed, respectively. A tendency to fit a Schultz-Flory distribution is apparent in Figure 7b. The addition of COz also causes a marked decrease of branched higher products, specifically isobutyl alcohol and isopentyl alcohol, whereas the productivity to straight-chain primary alcohols is not significantly affected. Such data suggest that two distinct mechanisms for HA formation are active over our catalyst: one is responsible for deviations from a Schultz-Flory distribution, results in formation of branched-chain alcohols, and is inhibited by C02 addition; the other mechanism leads preferentially to linear alcohols and is apparently unaffected by COP.
It is also worth noticing that the presence of carbon dioxide in the feed causes greater amounts of water to be produced via the shift reaction but reduces the formation of COz: based on the kinetic parameters of Table 111, and for the conditions of Figure 5, the net COz production would equal zero when 11% COz is added to the feed. For Hz/CO = 2/1, no COPformation would correspond to a feed COz content of about 6.5%. Effect of (H,/CO)O Ratio. Inspection of Figure 6 shows the presence of maxima in the yields of CH30H, HA, and COz as the feed molar ratio of the two main reactants is varied. The optimal Hz/CO ratio for methanol is located at about 2/1, as expected. The maximum in the production of COz is directly related to the maximum in the HA formation. For the higher alcohols, the optimum is located slightly above Hz/CO = 1/1 and apparently results from a balance between two distinct effects of the hydrogen/ carbon monoxide ratio on the rate of alcohol formation. According to eq 7, such a rate is enhanced by growing partial pressures of methanol but is depressed by water. The value of the H2/C0 ratio corresponding to the highest methanol partial pressure is two, as dictated by stoichiometry and confirmed by the data of Figure 6. On the other hand, removal of water via the shift reaction is favored by low Hz/CO ratios. Therefore, the optimum Hz/CO ratio for producing alcohols shall be less than two. Also, it will be smaller in those conditions where water inhibition is stronger. On assuming that chemical equilibrium is achieved by both reactions 1 and 3 and neglecting reaction 5, it is possible to obtain an analytical expression for the local optimum Hz/CO ratio, i.e., the ratio which maximizes the rate of higher alcohol formation according to eq 7. The resulting equation is (HZ/CO)OPT=
+ 14R + RZl1/’ 1 - 5R + [l + 14R + R2I1” 5-R - [l
(14)
Equation 14, which is represented in Figure 8, cannot be used directly to predict the integral results of Figure 6. It shows, however, that the point optimum value for Hz/CO decreases with increasing local COz concentrations and Kw values (i.e., water inhibition) and with smaller equilibrium constants of the direct water gas shift reaction. Process Implications. Concerning the Hz/CO feed ratio, while the kinetic analysis indicates an optimum value of about 1/1,determination of the economical optimum in an industrial process for the coproduction of methanol
2128 Ind. Eng. Chem. Res., Vol. 26, No. 10, 1987
and higher alcohols necessarily involves a global evaluation of the process. Particularly, upstream of the reactor, the process stages where the synthesis gas is prepared ought to be considered; downstream, the stages effecting the separation of the liquid products are also affected by the H2/C0 ratio, which determines the water content in the reactor effluents. Notice that a low Hz/CO ratio is beneficial under this respect, since it results in a more anhydrous final product. Also, it reduces the formation of hydrocarbons but poses more demands on the catalyst, due to greater risks of carbon deposition and may require more expensive construction materials in order to avoid formation of carbonyl metals. The presence of carbon dioxide in the feed involves also some relevant questions. In fact, for given process conditions the rate of alcohol formation would become maximum in the limit of total C 0 2 removal from the reactor feed (see Figure 5). The COPaddition also results in a less favorable HA/CH,OH weight ratio and in a reduced content of branched alcohols. Removal of carbon dioxide, however, requires a separation unit inserted in the synthesis loop (Di Pietro et al., 1980) and poses the problem of how to reutilize the removed carbon dioxide, as this is formed in considerable amounts. Figures 4-6 show that high productivities of higher alcohols are associated with high productivities of C02. A possible utilization would involve recycling the C02 removed from the synthesis loop to the stage of syngas preparation, which would also be helpful in reducing the Hz/CO ratio of the syngas. Finally, it ought to be recalled that the COz content of the reactor feed affects also the water present in the reaction products, thus influencing the costs of the separation and the value of the final products as well. Removing COP from the synthesis loop enhances the extent of the direct shift reaction, too. This makes the overall reaction scheme even more exothermic both because it increases the rate of the higher alcohol synthesis and because of its own heat of reaction. Therefore, removal of COz may eventually lead to more stringent requirements for the control of reactor temperature, since it is well-known that high temperatures favor the selectivity to hydrocarbons. Recent published process schemes typically include a stage for COz removal from the feed stream to the synthesis reactor (Quarderer, 1986; Supp, 1986; Hilsebein, 1986; Ohno et al., 1986).
Acknowledgment Financial support from Snamprogetti is gratefully acknowledged, We thank Prof. G. Buzzi-Ferraris for providing his nonlinear regression program and Dr. D. Sanfiiippo of Snamprogetti for valuable discussions and critical suggestions.
Nomenclature DH20 = water formed in excess of reactions 1-4 F = total molar flux, mol/h F, = molar flux of species i, mol/h HA = pseudoalcohol with N,carbon atoms k , = kinetic constant in rate expression for reaction i K = constant in rate equation for reaction 1 or 3, related to the corresponding equilibrium constant by eq 12
K,, = chemical equilibrium constant K , = kinetic parameter associated with H z O inhibition, eq 7, atm-’
K , = ratio of fugacity coefficients N = number of C atoms in alcohols
N, = average carbon number of oxygenated products HA
OL = pseudoolefin with N , carbon atoms p = partial pressure, atm P = total pressure, atm
r = rate of reaction, mol/(g h) R = group of variables defined by eq 15, dimensionless S = sum of squares of residuals, eq 13 T = temperature, O C W N = weight fraction of CNHZAVflOH W , = catalyst load, g Yl, = outlet mole percent of species i in run j Greek Symbols = characteristic parameter of Schultz-Flory molecular weight distribution E l = F,/FO Superscripts 0 = at reactor inlet OPT = optimal for higher alcohol production = calculated value Registry No. K,O, 12136-45-7;CO, 630-08-0;CH,OH, 67-56-1; CHSCHO, 75-07-0; CzH,OH, 64-17-5; CzH5CH0, 123-38-6; iC 3 H 7 0 H , 67-63-0; n - C 3 H 7 0 H ,71-23-8; i-C3H,CH0, 78-84-2; iC4HgOH,78-83-1; n-C4HgOH, 71-36-3; Z C5-OH, 30899-19-5; Z Cp,-OH, 25917-35-5; Z C,-OH, 53535-33-4;Z Cs-OH, 29063-28-3; zinc oxide, 1314-13-2;chromium oxide, 11118-57-3;CzHs,74-84-0; N
H&=CHz,
74-85-1; COZ, 124-38-9.
Literature Cited Anderson, R. B.; Feldman, J.; Storch, H. H. Znd. Eng. Chem. 1952, 44, 2418. Courty, P.; Durand, D.; Freund, E.; Sugier, A. J . Mol. Catal. 1982, 17, 241. Courty, P.; Arlie, J. P.; Convers, A.; Mikitenko, P.; Sugier, A. Hydrocarbon Process. 1984, Nov, 105. Di Pietro, R.; Paggini, A.; LaganA, V. Brev. Ital. 22 116, 1980. Figueras, F.; Nohl, L.; De Mourges, L.; Trambouze, Y. Trans. Faraday SOC.1971, 67, 1155. Forzatti, P.; Pasquon, I.; Villa, P. L.; Vita, G. Riu. Combust. 1984, 38, 207. Frohlich, P. K.; Lewis, W. K. Ind. Eng. Chem. 1928,20, 285. Frohlich, P. K.; Cryder, D. S.Ind. Eng. Chem. 1930, 22, 1051. Graves, G. D. Znd. Eng. Chem. 1931,23, 1381. Greene, M. J. Chem. Eng. Prog. 1982, 8, 46. Henrici-OlivB, G.; OlivB, S.In T h e Chemistry of the Catalyzed Hydrogenation of Carbon Monoxide; Springer: Berlin, 1984. Hilsebein, W., paper presented at the 1986 World Methanol Conference, Frankfurt, Dec 1986. Hindmarsh, A. C. ACM-Signum Newslett. 1980 15, 10. Hofstadt, C. E.; Schneider, M.; Bock, 0.;Kochloefl, K. Znt. S y m p . “Scientific Bases for the Preparation of Catalysts”, Presented at the 3rd International Symposium, Lovain-la-Neuve, Belgium, 1982. Klier, K. ”New Developments in the Synthesis of Light Alcohols“, In Catalysis of Organic Reactions; Moser, W. R., Ed.; Marcel Dekker: New York, 1980 and references therein. Klier, K.; Chatikavanij, V.; Herman, R. G.; Simmons, G. W. J . Catal. 1982, 74, 343. Mazanek, T. J. J . Catal. 1986, 98, 115. Mears, D. E. Znd. Eng. Chem. Process Des. Dev. 1971, 10, 541. Moe, J. Chem. Eng. Prog. 1962, 58, 33. Morgan, G. T.; Hardy, D. V. N.; Procter, R. H. J . Sot. Chem. Ind., London, Trans. Commun. 1932, 51, 1T. Natta, G.; Mazzanti, G.; Pasquon, I. Chim.Ind. (Milan) 1955, 37, 1015. Natta, G.; Colombo, U.; Pasquon, I. In Catalysis;Emmett, P. H., Ed. Reinhold: New York, 1957; Vol. V, p 131. Ohno, T.; Yoshimoto, M.; Asslineau, L.; Courty, P.; Travers, P., paper presented at the 78th Spring National AIChE Meeting, New Orleans, April 1986. Paggini, A.; Sanfilippo, D., paper presented at the 78th Spring National AIChE Meeting, New Orleans, April 1986. Pasquon, I. Chim.Znd. (Milan) 1984,66, 700 and references therein. Pillai, G. C.; Wei, T. C. J.; Stiles, A. “Synthesis of Methanol and Higher Alcohols from CO, COPand H,. An interim Report”, 1981; Center for Catalytic Science and Technology, University of Delaware. Quarderer, G., paper presented at the 78th Spring National AIChE Meeting, New Orleans, April 1986. Runge, F.; Zepf, K. Brennst.-Chem. 1954, 35, 167. Sanfilippo, D., personal communication, 1986.
I n d . E n g . C h e m . R e s . 1987, 26, 2129-2134 Smith, K. J.; Anderson, R. B. Can. J. Chem. Eng. 1983, 61,40. Smith, K. J.; Anderson, R. B. J. Catal. 1984,85,428. Supp, E.,paper presented at the 78th Spring National AIChE Meeting, New Orleans, April 1986. Taylor, R. J . Chem. SOC.(London) 1934,1429. Vedage, G. A.; Himelfarb, P.; Simmons, W. G.; Klier, K. Prepr. Pap.-Am. Chem. SOC.Pet. Chem. Div. 1983,28,1261.
2129
Villa, P. L.; Forzatti, P.; Buzzi-Ferraris, G.; Garone, G.; Pasquon, I. Ind. Eng. Chem. Process Des. Dev. 1985,24,12. Villa, P. L.; Del Piero, G.; Cipelli, A.; Lietti, L.; Pasquon, I. Appl. Catal. 1986,26,161.
Received f o r review July 23, 1986 Accepted June 2, 1987
A Simplified Flash Procedure for Multicomponent Mixtures Containing Hydrocarbons and One Non-Hydrocarbon Using Two-Parameter Cubic Equations of State Bjarne H. Jensen and Aage Fredenslund* Znstituttet f o r K e m i t e k n i k , Technical University of Denmark, DK-2800 L y n g b y , Denmark
An economical two phase (P,T) flash procedure for cubic equations of state containing two parameters (a and b ) is developed. T h e geometric mean is used as the mixing rule for the a parameter for hydrocarbon-hydrocarbon interactions, but a deviation parameter is used for non-hydrocarbonhydrocarbon interactions. When there is only one non-hydrocarbon present, this permits reduction of the flash problem t o solving five equations in five unknowns, irrespective of the number of components in the mixture. For multicomponent flash calculations, this implies substantial savings in computer time and storage requirements compared with standard flash procedures. T h e new procedure is advantageous t o use in, e.g., simulation of COSand N2 flooding of oils in reservoirs.
For hydrocarbon-hydrocarbon interactions, the oil characterization procedure of Pedersen et al. (1985) uses the geometric mean to describe the a parameter in the Soave-Redlich-Kwong equation of state (SRK-EOS) (Soave, 1972). The deviation parameter (termed Ki,) is equal to zero. The correlations used to describe the a and b parameters for the hydrocarbon fractions are developed such that K i j = 0 for hydrocarbon-hydrocarbon interactions gives the best results for the thermodynamical properties of the reservoir fluids. When Ki, = 0 for all combinations of i and j , Michelsen (1986) has shown that the two-phase (P,T)flash problem reduces to solving three equations in three unknowns, for any multicomponent mixture. This significantly reduces computational costs and storage requirements. In some important practical applications, significant amounts of non-hydrocarbons are present. Examples of this are oil reservoir flooding with CO, or N2. In these cases, the characterization procedure of Pedersen et al. (1985) still permits Ki; = 0 for all hydrocarbon-hydrocarbon interactions. However, for acceptable results, the C02-hydrocarbon or the N2-hydrocarbon interactions must be different from zero. This paper formulates an economical flash algorithm for cases where significant amounts of non-hydrocarbons are mixed with multicomponent hydrocarbon mixtures. The flash algorithm is outlined by using the Peng-Robinson equation of state (Peng and Robinson, 1976) as an example, but it can be used with any other two-parameter cubic equation of state, such as the SRK-EOS. Peng-Robinson Equation of State The Peng-Robinson equation of state, eq 1 (Peng and Robinson, 1976), has two parameters, a and b, defined as shown in eq 2, where the critical coefficients, a, and b,, are 0888-5885/87/2626-2129$01.50/0
given by eq 4 and obtained from the condition stated by eq 5. a p = - -RT (1) u - b (U + b)U + b(U - b) a =
m ( T ) = (1 +
a,T,2R2m(T ) b = -b C T 3 (2) p, PC (0.37464 1.54226~0.26990')(1 - (T/T,)'/'))' (3)
+
a, = 0.45724 b, = 0.07780 [dP/dU],= [d2P/du2],= 0
(4) (5)
For mixtures, the EOS parameters are evaluated through mixing rules, as shown in eq 6 and 7. The parameter Ki, describes the deviation from the geometric mean. N N
a =
C Cnixj(aiaj)'/2(l- Kij) i=1;=1
(6)
N
b = Cxibi i=l
(7)
The equilibrium relations for multicomponent systems are given in terms of fugacity coefficients, which are calculated after having solved the compressibility equation
where the largest or the smallest root is chosen, depending on the phase considered.
0 1987 American Chemical Society
